Secondary growth in dicots & gymnosperms occur by

Question

$\begin{array}{1 1}(A)\;\text{Formation of vascular rays}\\(B)\;\text{Thickening of tracheary elements}\\(C)\;\text{Formation of meristematic cells in the vascular region}\\(D)\;\text{Development of meristematic cells in vascular & cortical regions}\end{array}$